1. Field of the Invention
The invention relates to a modified, thermosetting urea-formaldehyde resin composition useful as a binder for making fiber mats, to fiber mats prepared using the modified urea-formaldehyde resin composition as a binder, and to a process of preparing the mats using the binder. In particular, the invention relates to a fiber mat binder composition comprising a thermosetting urea-formaldehyde resin (UF) modified by the addition of a water soluble styrene-maleic anhydride copolymer (SMA).
2. Background of the Invention
Glass fiber mats and fiber mats made from other synthetic fibers are finding increasing application in the building materials industry, as for example, in composite flooring, in asphalt roofing shingles, or siding, replacing similar sheets traditionally made using wood, cellulose or asbestos fibers.
Fiber mats, and especially glass fiber mats, usually are made commercially by a wet-laid process, which is carried out on what can be viewed as modified papermaking machinery. Descriptions of the wet-laid process may be found in a number of U.S. patents, including U.S. Pat. Nos. 2,906,660, 3,012,929, 3,050,427, 3,103,461, 3,228,825, 3,760,458, 3,766,003, 3,838,995 and 3,905,067. In general, the wet-laid process for making glass fiber mats comprises first forming an aqueous slurry of short-length glass fibers (referred to in the art as "white water") under agitation in a mixing tank, then feeding the slurry onto a moving screen on which the fibers enmesh themselves into a freshly prepared wet glass fiber mat, while excess water is separated therefrom.
Unlike natural fibers such as cellulose or asbestos, glass fibers do not disperse well in water. To overcome this problem, it has been the industry practice to provide suspending aids for the glass fibers. Such suspending aids or dispersants usually are materials which increase the viscosity of the aqueous medium. Suitable dispersants conventionally employed in the art include polyacrylamides, hydroxyethyl cellulose, ethoxylated amines and amine oxides. Other additives such as surfactants, lubricants and defoamers also have conventionally been added to the white water. Such agents, for example, further aid the wettability and dispersion of the glass fibers. Experience has shown that such additives also often influence the strength of the wet glass fiber mat.
The fiber slurry deposited on the moving screen or cylinder is processed into a sheet-like fiber mat by the removal of water, usually by suction and/or vacuum devices, and is followed by the application of a polymeric binder to the mat. In the manufacture of glass fiber mats, a high degree of flexibility and tear strength is desired in the finished mat in addition to primary dry tensile and wet tensile properties. A binder composition is therefore used to hold the glass fiber mat together. The binder composition is impregnated directly into the fibrous mat and set or cured immediately thereafter to provide the desired mat integrity. The binder composition is applied to the mat by soaking the mat in an excess of binder solution or suspension, or by impregnating the mat surface by means of a binder applicator, for example, by roller or spray. The primary binder applicator for glass mat machines has been the falling film curtain coater. Suction devices often are also utilized for further removal of water and excess binder and to ensure a thorough application of binder through the glass fiber mat. A widely used binder is based on a urea-formaldehyde resin commonly fortified with an emulsion polymer. UF resins have been employed because they are relatively inexpensive. In addition to mat strength properties which the binder composition imparts to the ultimately cured mat, the binder also functions to improve the strength of the uncured, wet-laid mat as it is transported from its initial formation into and through the curing oven. Such incipient pre-cured strength is needed to avoid process delays and shutdowns caused by breaks in the endless mat.
Thus-incorporated binder is thermally cured, typically in an oven at elevated temperatures. Generally, a temperature in the range of about 200 to 250.degree. C. is used during curing. Normally, this heat treatment alone will effect curing of the binder. Catalytic curing, such as is accomplished with addition of an acid catalyst (for example, ammonium chloride or p-toluene sulfonic acid), generally is a less desirable, though an optional, alternative.
Because glass fiber mats made with a binder consisting essentially of a UF resin often are brittle, or because the strength properties of the mats may deteriorate appreciably subsequent to their preparation, especially when the mats are subjected to wet conditions, UF resin binders have commonly been modified by formulating the UF resin with cross-linkers and various catalyst systems or by fortifying the UF resin with a large amount of latex (emulsion) polymer, usually a polyvinyl acetate, vinyl acrylic or styrene-butadiene. Certain latexes can provide increased wet tensile strength and tear strength. The use of styrene-butadiene latex-modified, urea-formaldehyde resin compositions as a binder for glass fiber mats is disclosed, for example, in U.S. Pat. Nos. 4,258,098; 4,560,612 and 4,917,764.
The present invention is directed to an improved (modified) thermosetting urea-formaldehyde resin-based binder composition useful as a binder for making fiber mats, particularly glass fiber mats, to fiber mats prepared using the modified urea-formaldehyde resin composition as a binder, and to a process of preparing the fiber mats using the binder. In particular, the invention relates to a fiber mat binder composition comprising a thermosetting urea-formaldehyde resin (UF) modified by the addition of water soluble styrene-maleic anhydride copolymer (SMA).